1. Field of the Invention
The present invention is directed to heat sinks and in particular to a spring clip for applying a compressive force between an electronic component and a heat sink.
2. Description of the Related Art
Electronic components secured to circuit boards, as for example power transistors and processors, generate significant amounts of heat which must be dissipated for the components to operate reliably. Some of the heat can be dissipated through the leads of the components. Much of the excess heat, however, is released into the ambient air. It is known in the art that cooling of such electronic components can be improved by thermally coupling the component to a finned or pinned heat sink.
Traditionally, electronic components are mounted to heat sinks by passing bolts through holes defined in the heat sink and the component and securing the bolts with separate nuts. In other configurations, the heat sinks employ tapped holes or threaded fasteners pressed into the heat sink which accommodate separate mounting bolts. In still other arrangements, threaded studs are pressed into the heat sink and the components are mounted thereto using separate nuts.
These conventional arrangements present significant disadvantages. First, mounting holes in electronic components are not typically located in the center of the component and thus, the compressive force imparted to one end of the component by the mounting hardware often causes the opposite end to lift upwards and out of thermal contact with the heat sink. Second, the installation and removal of separate hardware is time consuming and not readily performed automatically using machines, thereby increasing the labor and manufacturing costs. Finally, the trend in the electronic industry has been to eliminate mounting holes in component packages thereby rendering traditional hardware impracticable or obsolete.
In order to overcome these disadvantages several spring clip designs have been developed that mount the component to the heat sink without the use of hardware. For example, the heat sink may be stamped from aluminum with integral clipping mechanisms. Aluminum, however, does not exhibit mechanical properties sufficient to maintain deflection forces over time at increased temperatures and the condition on stamping relatively small sizes limits the applicability of such design.
The size limitation has been overcome by using separate stamped aluminum clips that are riveted to a stamped or extruded heat sink. This alternative configuration, however, does not solve the mechanical problems associated with manufacturing the spring clip using aluminum and requires an additional manufacturing step that increases the overall cost.
It is also known to use stamped steel clips attached to mounting features extruded into a specific heat sink. These clips are made from steel and thus overcome the mechanical problems associated with aluminum but have limited applicability in that the steel clips are only suitable for use with specific heat sinks.
The design of these stamped steel clips has also been modified to include bayonet type retainers which pass through a hole in the heat sink and, in some cases, through a hole in the component. Although not limited to use with specific heat sinks, this spring clip configuration sacrifices spring width for the retention features resulting in relatively large extreme fiber stresses at the union of the bayonet arms to the main body of the spring clip. Thus, such spring clips are limited in the amount of force and range of deflection which may be achieved prior to plastic deformation. Another disadvantage of this particular design is the inability to mount components to opposite surfaces of a heat sink utilizing the same mounting features.
Still other steel clips have been developed having one end which passes into or through a horizontal slot in the heat sink. This configuration overcomes the stress problems associated with bayonet type designs but are prone to excessive rotation under load due to manufacturing and design tolerances, and thus are limited to use with relatively thick walled heat sinks. Moreover, this construction is extremely sensitive to variations in wall thickness and slot tolerances. In addition, the horizontal orientation of the mounting slots impedes the conduction of heat from the device area below the mounting slots to the finned area above the mounting slots thereby increasing the thermal resistance of the heat sink.